The Handy Chemistry Answer Book (The Handy Answer Book Series) ( PDFDrive )

What is entropy? Entropy is a measure of the total number of microstates in a system. There have been two widely used definitions of entropy, which were suggested by Ludwig Boltzmann and J. Willard Gibbs. We’ll just look at the one specified by Boltzmann, since it’s a little more straightforward to understand. The equation for Boltzmann’s definition of entropy is: S ϭ kb ln( ) In this equation, kb is Boltzmann’s constant, and is the number of microstates ac- cessible to a system. To get an idea of how entropy works, consider the example of rolling one or more six-sided die. On the first roll, there are six possible outcomes, so the entropy associated with rolling one die is kbln(6). If we roll two dice, there are 62 ϭ 36 possible outcomes, and the associated entropy is kb ln(36). For three, it’s 63 ϭ 216 possible outcomes, and the associated entropy is kbln(216). As you can see, the number of outcomes for statis- tically independent events grows very rapidly (exponentially) with the size of our system, which is also true for molecules. By taking the logarithm of the number of outcomes, we make the entropy scale linearly with system size. While the number of possible out- comes/configurations grows exponentially with system size, the entropy grows linearly, which means that if we double the system size we double the entropy. This property makes entropy fall into a category of variables known as extensive variables, which just means that they scale with the size of a system in this simple way. What is the second law of thermodynamics? There are several different statements of the second law of thermodynamics, but they are all centered on the idea of identifying what things can happen spontaneously in nature. One formulation of the second law states that, for a closed system, the entropy of the system can only increase or remain the same. In plain language, this says that na- ture favors having more accessible configurations or arrangements. It’s why, for exam- ple, a drop of ink in water tends to spread out to fill its accessible volume but won’t spontaneously reform a drop of ink. Another statement of the second law says that heat cannot spontaneously flow from a colder body to a warmer body. Work would have to be done for this to happen, which would imply the process was not spontaneous. What is the third law of thermodynamics? The most common statement of the third law of thermodynamics is that the entropy of a perfectly crystalline system approaches zero as the temperature of the system ap- proaches zero. (Recall from “Macroscopic Properties: The World We See” that a perfect crystal is a regularly ordered lattice of atoms that exist in a repeating pattern in three dimensions with no defects or irregularities in the lattice.) This is equivalent to saying 138 that a perfectly crystalline system has only one accessible state as the temperature ap-

proaches zero. In truth, this isn’t always strictly true since there can be multiple low- PHYSICAL AND THEORETICAL CHEMISTRY energy states that all have the similar energy, but let’s ignore this for now. What effects cause deviations from the ideal gas law? Deviations from the ideal gas law occur due to intermolecular forces between the gas particles as well as the fact that gas particles do actually occupy volume. There is a mod- ified version of the ideal gas law (see “Atoms and Molecules”), called the van der Waals equation of state, that uses constants specific to each molecule or atom to adjust for these factors. Deviations from ideal gas law behavior become more important at rela- tively high pressures and/or low temperatures. What is the average kinetic energy of a molecule? The average kinetic energy of a molecule is closely related to the temperature of its en- vironment, and this determines how fast the molecule is moving. On average, the speed of any molecule is close to 300 m/s, which is equivalent to covering the distance of a few football fields every second! The other important thing to realize, though, is that colli- sions with other molecules are constantly causing changes in direction, which slows the overall distance in a given direction that a molecule travels. What is an ideal solution? An ideal solution describes a solution of dilute solute particles that do not interact with one another. It is very similar to an ideal gas, except that instead of empty space occu- pying the space between the gas particles, a weakly interacting solvent occupies the space between solute particles. What is osmosis? Osmosis is the movement of solvent molecules in a solution to establish an equal con- centration of solute throughout the solution. Solvent molecules move from areas of low-solute concentration to areas of high-solute concentration, which tends to remove any gradient in solute concentration. In osmosis, solvent molecules move from areas of low solute concentration to areas of high solute concentration 139 through a permeable membrane to equalize the solute concentration on either side.

What is an isothermal process? An isothermal process is a process in which the temperature remains constant through- out the process. What is an isobaric process? An isobaric process is a process that is carried out at a constant pressure. What is an adiabatic process? An adiabatic process is a process in which no heat is exchanged with the surroundings. What is an isochoric process? An isochoric process is a process that takes place at a constant volume. KINETICS What is the transition state for a reaction? The transition state of a chemical reaction is the highest energy structure through which the reactant molecule(s) must pass to complete the reaction. Since this is the highest energy point along the path of the reaction, this configuration is the most “dif- ficult” to reach along the reaction path and thus the energy barrier to reach the transi- tion state limits how quickly the reaction can proceed. What is the rate constant for a reaction? The rate constant for a chemical reaction is a quantity that describes how rapidly the re- action proceeds. Rate constants can have different units, depending on how many mole- cules are involved in the reaction. Consider a simple reaction where a single molecule of a species A becomes a molecule of species B. The rate of the reaction will depend on the concentration of species A (denoted [A]) present, and the rate constant (k) for this reac- tion. The rate equation for this reaction would be: Reaction rate ϭ k[A] This tells us that the reaction rate depends only on the concentration of A, and that the reaction rate will increase as the concentration of A is increased. In truth, the reac- tion rate also depends on the temperature, pressure, and perhaps other factors as well, but these are all bundled into the rate constant, k. How is the rate of a reaction affected by temperature? The rate of a chemical reaction will generally increase with increasing temperature. This 140 is because a higher temperature translates into a higher average energy per molecule,

which makes it easier for molecules to surmount the energetic barrier to the reaction. In PHYSICAL AND THEORETICAL CHEMISTRY terms of how this fits into the rate equation, the rate constant k depends on temperature, and k almost always (but there are exceptions) increases with increasing temperature. FASTE R THAN A S P E E D I N G WAVE How fast does light travel? In a vacuum, the speed of light is about 3 ϫ 108 meters per second, which is very, very fast. That’s so fast that a beam of light could travel around the whole world in only about 0.13 seconds! It’s interesting to consider just how far away from the Earth stars really are. After the Sun, the next closest star to the Earth is over four light years away (a light year is the dis- tance light travels in one year). This means the next closest star is over 20,000,000,000,000 miles away. Because light must reach our eyes for us to see anything, if that star exploded, we wouldn’t see it until over four years after it actually happened! Can anything travel faster than the speed of light in a vacuum? No, or at least it’s presently thought that this would be impossible. It’s interesting, though, that there have been a couple of experiments in recent years which have ob- served particles called neutrinos moving faster than the speed of light. Even the scien- tists who carried out these experiments questioned the results, however, and they encouraged others to try to confirm their results or to find where a mistake might have been made in their measurements. In the end, they did find out that these results were caused by a mistake; there was a loose electrical cable that caused enough error to in- validate the results. Is the speed of light always the same? This photo combines two images: one of a stick 141 partly submersed in a glass of water (A), and the The speed of light actually depends on same picture without water in the glass (B). The what material the light is moving through! stick seems bent in A because light is refracted as it Each material has a property called an leaves the water and is perceived by your eyes. index of refraction. From the index of re- fraction, we can calculate the speed of light passing through a material from the fol- lowing equation: v ϭ c/n In this equation, c is the speed of light in a vacuum (about 2.998 ϫ 108 m/s), n is the index of refraction of the material in question, and v tells us the velocity of light in the material.

Electromagnetic radiation consists of electric and magnetic fields that run perpendicular to one another. The wavelength of light is the distance measured between wave crests. What are the wavelength and frequency of light? The light we see is a form of energy called electromagnetic radiation. It may sound like something complicated, but it’s nothing too exotic or unfamiliar. Everything you see is because of electromagnetic radiation. Electromagnetic radiation consists of perpendic- ular electric and magnetic fields that oscillate in amplitude. The number of times per second that the fields oscillate is the frequency of the radiation. This is measured in Hertz (Hz), or inverse seconds. The wavelength is the distance that the light travels through space during one oscillation of the electric or magnetic field. What is the electromagnetic spectrum? The electromagnetic spectrum describes the entire range of frequencies (or wave- lengths) possible for electromagnetic radiation to have. In principle, the spectrum is practically infinite, though there are limitations on how high or low of frequencies we can practically achieve and work with. On the high end of the frequency spectrum are usually gamma rays, with frequencies of around 1020 Hz, while on the low end are “ex- tremely low frequencies” of only a few Hz. Electromagnetic Spectrum Type Frequency (Hz) Wavelength (cm) Radio Ͻ 3 ϫ 1011 Ͼ 10 Microwave 3 ϫ 1011– 1013 10 – 0.01 Infrared 1013 – 4 ϫ 1014 0.01 - 7 x 10–5 Visible 4 – 7.5 ϫ 1014 7 x 10–5 – 4 x 10–5 Ultraviolet 1015 – 1017 4 x 10–5 – 10–7 X–rays 1017 – 1020 10–7 – 10–9 142 Gamma Rays 1020 – 1024 Ͻ 10–9

How is the frequency of electromagnetic radiation related to its energy? PHYSICAL AND THEORETICAL CHEMISTRY The frequency of a photon is related to its energy, E, by a pretty straightforward equation: Eϭh In this equation, h is Planck’s constant, which has a value of 6.626 ϫ 10–34 J•s. The frequency term, , is the frequency of the radiation in Hz. As we can see from this equa- tion, electromagnetic radiation with higher frequency has higher energy. BIG FREAKING LASERS What is spectroscopy? Spectroscopy is a branch of science associated with using light to study transitions be- tween energy levels. Not all scientists who use spectroscopy (spectroscopists) are phys- ical chemists, though physical chemists (and physicists) are typically the people who develop new spectroscopic methods and experimentally investigate the details of how light interacts with matter. Data collected in spectroscopic experiments is typically pre- sented as some response of an atomic or molecular system as a function of frequency/ wavelength or time. When the response is plotted as a function of frequency/wavelength, it is called a spectrum. What are Fraunhofer lines? When scientists first began observing the spectrum of light reaching Earth from the Sun, the spectrum contained many dark lines, which indicated that light of certain wavelengths wasn’t present in the sunlight. These are now called Fraunhofer lines (named after their discoverer), and they are caused by the elements in the outer at- mosphere of the Sun absorbing certain wavelengths of light, which prevents those wave- lengths from reaching the Earth. Understanding that Fraunhofer lines were caused by atomic absorptions was one of the earliest examples of atomic spectroscopy. What are ground and excited electronic states? The ground electronic state of an atom or molecule is the lowest energy electronic state. Excited states are any electronic states that have a higher energy than the lowest energy electronic state. How can light cause transitions An example of a typical spectrum plotting ab- 143 sorbance versus wavelength. between energy levels? Light comes in discrete units called pho- tons, and each photon has a particular en-

A laser beam is simply a beam of light that has been intensified by stimulating the emission of photons. They can be used for many purposes, ranging from cutting metal to delicate surgery to aiding scientists with complex measurements. ergy associated with it. When a photon’s energy matches the energy spacing between two energy levels in an atom or molecule, it can cause a transition between these energy lev- els. This results in absorption of the photon, which transfers its energy to the atom or molecule. For example, the energy spacing between the ground and first excited elec- tronic states of a hydrogen atom is 1.64 ϫ 10–18 J which corresponds to a photon fre- quency of 2.47 ϫ 1015 Hz. So photons with this frequency can excite the electron in a hydrogen atom from the ground to first excited electronic state. What is a laser? A laser is a light source that emits light amplified through the stimulated emission of pho- tons. The acronym, LASER, actually stands for Light Amplification by Stimulated Emis- sion of Radiation. Lasers come in many shapes and sizes. Some can fit in your pocket, and some are huge and take up entire rooms. Some emit pulses of light, while others emit continuous beams of light. Since there are so many types of lasers, it’s not surprising that they find wide-ranging applications from simply pointing at a screen during a pre- sentation to carrying out complex measurements in physics and chemistry experiments. Why are lasers useful for physical chemists? Chemists use lasers to study how molecules interact with light. In some cases, a chemist 144 may want to know how a molecule reacts when a pulse of light is used to excite it. In

Can lasers be dangerous? PHYSICAL AND THEORETICAL CHEMISTRY Definitely. Many lasers used in modern chemistry laboratories are powerful enough to cause a person to go blind after only a fraction of a second of direct exposure to the eye. Some are even so powerful that they can burn or ignite ob- jects placed in their path. Laser pointers you can buy in the store are not this pow- erful, however, so you don’t have to worry about a laser you personally own being quite this dangerous. You should still definitely avoid shining them in your eye, though, because they can be damaging. other cases, lasers can be used to gain information about the structure of the molecule. One of the reasons lasers are good for these purposes is that they can provide pulsed light to gain information about how molecules are changing over time. Another is that many “tricks” exist for controlling and manipulating the wavelength of the light produced by a laser, making them versatile light sources. What is the biggest laser in the world? The largest laser in the world is located at the Lawrence Livermore National Laboratory in Livermore, California. This laser is so large that it covers the size of three football fields! The scientists who use this giant laser for their research are hoping to show that a nuclear fusion reaction (see “Nuclear Chemistry”) can be controlled and used as a source of en- ergy. If that’s possible, it could revolutionize the way power plants make energy. OTHER SPECTROSCOPY What is microwave spectroscopy? Microwave spectroscopy, as the name implies, is spectroscopy carried out using electro- magnetic radiation in the microwave region of the spectrum (0.3 to 300 GHz). The en- ergies associated with microwaves are relatively low, and these energies are typically a good match for energy-level spacing between the different rotational levels of molecules. Thus, microwave spectroscopy is typically used to study the rotational energy levels of molecules. The rotational energy levels of molecules are typically studied in the gas phase. What is infrared spectroscopy? 145 Infrared spectroscopy is carried out using somewhat higher energy electromagnetic ra- diation (300 GHz to 400 THz) than microwave spectroscopy. The infrared region of the spectrum is usually a good match for the vibrational energy-level spacing in molecules, so infrared spectroscopy is typically used to study the vibrational energy levels of mol-

How does RADAR work? RADAR, which stands for RAdio Detection And Ranging, works by sending out electromagnetic radiation, allowing it to bounce off of objects, and receiving it again after it has been reflected. The RADAR system measures things like how long it took for the signal to make it back, how the frequency of the signal has changed, and how the strength of the signal has changed. From this information, the RADAR system can “see” where the objects that reflected the light are located. This can also be used to determine the speed of an object, such as when the police use a RADAR gun to track the speed of a vehicle. ecules. Vibrational spectroscopy can be used to study molecules in the gas, liquid, and solid phases, as well as molecules on surfaces. What is UV/Vis spectroscopy? Electromagnetic radiation in the ultraviolet and visible region of the spectrum (40 to 1000 THz) is higher in frequency (and thus energy) than that in either the microwave or infrared. This makes it a good match for the larger energy-level spacing associated with transitions between electronic energy levels. UV/Vis spectroscopy can be used to study molecules in any phase; however, it is most commonly used for liquid samples. What is Beer’s law? Beer’s law tells us how the amount of electromagnetic radiation absorbed by a sample is related to the concentration of the absorbing species. Beer’s law tells us that the ab- sorbance is equal to the length of the sample, l, times the concentration, c, of the ab- sorbing species in the sample, times the molar absorptivity coefficient, e, of the species. A ϭ elc In this equation A is absorbance, which is defined as the negative of the logarithm of the ratio of the intensity of light passing through a sample to that incident on it. Ba- sically, this gives us a measure of how much light a sample is absorbing and how much light is passing through it. What is fluorescence? Fluorescence is a process by which molecules that have absorbed light can re-emit light to release some of the energy they absorbed. For fluorescence to take place, a molecule must first absorb a photon of light, which causes an electron to be excited to a higher energy level. At the same time, this process will typically also cause some vibrational ex- citations to take place. Some of the energy associated with this absorption will be given 146 off through relaxation of the excited vibrational energy levels. For fluorescence to occur,

Why do “black” lights make white materials appear to glow? PHYSICAL AND THEORETICAL CHEMISTRY “Black” lights are lights that emit ultraviolet light at frequencies on the upper- edge, or higher, relative to what our eyes can see. Many objects can absorb these frequencies and then undergo fluorescence, giving off light at lower fre- quencies that our eyes can see. This is the reason “black” lights cause these ma- terials to appear to glow. the electronic excitation relaxes by emitting a photon of light. Some of the energy was 147 dissipated as the vibrational energy levels relaxed, so the photon that is emitted has less energy than the photon that was initially absorbed. Remember that less energy means a lower frequency, so the photons that are emitted have a lower frequency than those that were absorbed. What is mass spectrometry? Mass spectrometry is a method of chemical analysis that involves determining the mole- cular mass of charged particles by measuring the mass-to-charge ratio of an ionized mol- ecule or molecular fragment. There are several ways of performing mass spectrometry, but the general procedure involves making the sample into a vapor, ionizing the sample, and then detecting the ions that form in a way that separates them according to their mass-to-charge ratio. After being ionized, the molecules in the sample will often fragment into smaller ions, and these too are detected according to their mass-to-charge ratio. This technique can be useful for carrying out an accurate determination of the mass of a molecule as well as for obtaining structural information about molecules via their fragmentation patterns. It also allows the elemental composition of a sample to be determined. How does a microscope work? A microscope is all about lenses. The lens near the sample you’re looking at is called the objective lens, and this lens is responsible for collecting the light from the sample and fo- cusing it. Typically there will be a light under or behind the sample that provides the light used to view the sample. At the other end is another lens called the ocular lens, and the total magnification of the microscope is determined by multiplying the magnification of the objective lens by that provided by the ocular lens. The apparatus we typically think of when we think of a microscope is essentially just a big framework used to hold the lenses, the sample, and perhaps other optical devices used to improve the image of the sample. What is electron microscopy? An electron microscope is a microscope that uses a beam of electrons to produce an image of a sample (rather than using light like in a standard microscope). There are

several ways of obtaining an image, but the Simple optical microscopes work by magnifying im- original was the transmission electron mi- ages using lenses. croscope (TEM), which produces an image by passing an electron beam directly through the sample. Electron microscopes offer a significant advantage in resolution over traditional light microscopes. This is due to the fact that the wavelengths asso- ciated with electrons are much shorter than those associated with visible light. It possible to achieve resolutions of up to roughly 10,000,000 times magnification using electron microscopes, as compared to about 2,000 times magnification in the best light microscopes. What is electrical resistance? Electrical resistance describes how a material opposes an electric current through the material. The resistance is related to the applied voltage through the relationship: R ϭ V/I where V is the applied voltage and I is the current through the material. Typically the resistance is a constant, and thus the current will increase linearly with an increase in the applied voltage. This relationship is Ohm’s law. As you can see, a material with a higher resistance (R) will have a lower current (I) for a given applied voltage (V). What is a voltage? A voltage, or potential difference, is the difference in electric potential energy between two points. The voltage describes the amount of work that would need to be done, per unit charge, to move a charged object between the two positions. A voltage may be pre- sent due to a static electric field, to electric current flowing through a magnetic field, or due to magnetic fields that change over time. IF YOU CAN IMAGINE IT What is the focus of theoretical chemistry? Theoretical chemistry is a branch of chemistry that, as the name suggests, develops and applies theories to explain chemical observations and also to make predictions about things chemists cannot directly study by experiment. Theoretical chemists work on a wide variety of problems that cover pretty much all of the other branches of chemistry. Two of the major subfields within theoretical chemistry are electronic structure theory 148 and molecular dynamics.

What is electronic structure theory? PHYSICAL AND THEORETICAL CHEMISTRY Electronic structure theory is an area of theoretical chemistry that is focused on calcu- lating the arrangement and energies associated with configurations of electrons in mol- ecules. This can include predicting the structure of a molecule, the most probable arrangement of its electrons, its reactivity, and different excited states of the molecule. For reasons we won’t go into, this is not an easy task, and the electronic structure of molecules cannot be solved exactly, even with very powerful computers. Most theoret- ical chemists working in this area develop approximations for calculating the true elec- tronic structure of molecules and on testing these approximations against available experimental data to continue to improve existing methods. The electronic properties of molecules play a vital role in determining their stability and reactivity, so despite being challenging, it’s a problem well worth trying to solve. What molecular properties do theoretical chemists try to calculate? Theoretical chemists try to calculate pretty much every molecular property there is! If there’s a property we’ve discussed somewhere in this book, odds are that a theoretical chemist has worked on ways to calculate its value for atoms or molecules. How much error is associated with electronic structure theory calculations? There can be a pretty large amount of error associated with these calculations, and the main goal is to keep the errors as consistent as possible and to obtain properties via dif- ferences in calculations. For example, the energy of the metal–carbon bond in Cr(CO)6 would be calculated by comparing the energy calculated for Cr(CO)6 to that calculated for Cr(CO)5 and CO separated from one another at infinite distance. What is a molecular dynamics simulation? Molecular dynamics simulations are a computational model of a collection of molecules that interact with one another under a specified set of conditions (temperature, pressure, etc.). While electronic structure theory calculations typically only involve one or a few molecules, molecular dynamics simulations can involve hundreds or thousands of mole- cules all at once. The purpose of molecular dynamics simulations is typically to investigate how molecules interact and react while surrounded by a collection of other molecules. While the energies of individual molecules can be investigated by electronic structure the- ory, a molecular dynamics simulation allows theoretical chemists to also study how mol- ecules are influenced by their surroundings. These effects can be especially important in liquids, where solvent molecules can have a significant impact on reactivity. 149



NUCLEAR CHEMISTRY C H E M I STRY I N S I D E TH E ATO M 151 How is nuclear chemistry different than other types of chemistry? As the name implies, nuclear chemistry deals specifically with chemical events involv- ing the nucleus itself, while most other areas of chemistry involve rearrangements of the electrons. Nuclear chemistry is focused on radioactivity and the properties of nuclei, and it finds some of its most important applications in energy production, weapons, and medicine. What is an isotope? Isotopes are atoms with the same number of protons and electrons, but with different numbers of neutrons. The most important thing to keep in mind is that the number of protons determines the element we’re dealing with. In most areas of chemistry, this is enough to determine the reactivity of the atom, though in nuclear chemistry the num- ber of neutrons is also quite important in determining the nuclear processes an isotope can undergo. (Please refer back to the “Atoms and Molecules” chapter for additional in- formation on isotopes.) Do electrons, protons, and neutrons all have the same mass? Protons, neutrons, and electrons each have different masses. Electrons are, by far, the lightest of the three, with a mass of only about 1/2000th that of a proton or neutron. Pro- tons and neutrons have similar masses, with that of the neutron being just slightly higher than that of the proton. The masses of the three particles in kilograms are: Electron mass: 9.1094 ϫ 10–31 kg Proton mass: 1.6726 ϫ 10–27 kg Neutron mass: 1.6749 ϫ 10–27 kg

Nuclear decay processes can involve the release of electrons, protons, neutrons, or combinations of these basic particles. Are all isotopes stable? Not all isotopes of a given element are stable. For example, tin has twenty-two different known isotopes, ten of which are stable and twelve of which are unstable (though there is some debate about just how stable those ten are). Stable is, of course, a relative term. Usually when one says an isotope of an element is stable, it means that it has a decay half- life that is too long to be measured by current methods. There are some elements, such as technetium, radon, and plutonium that do not have any stable known isotopes. In fact, no elements with an atomic number of over 83 (i.e., more than eighty-three protons) have any known isotopes that are considered to be stable! What is an antiparticle, and what is antimatter? For most kinds of particles there is postulated to exist a corresponding antiparticle, which is of the same mass but an opposite charge. These antiparticles have only recently been observed in laboratory settings for the first time, and they are very difficult to iso- late and study experimentally. This is because particle and antiparticle pairs collide to generate photons of light in a process that annihilates the particle and antiparticle pair. Antiparticles are not well understood and are an active area of research related to nu- clear chemistry. Antimatter is just matter made up of antiparticles, in the same way that normal matter is made up of particles. There has been postulated to be an equal amount of matter and antimatter in the universe, though the observations made to date do not suggest this to be the case. This represents an unresolved dilemma that scientists hope to someday better understand. These types of fundamental, unresolved problems are a big part of the reason science is so interesting! What is a positron? A positron is the antimatter counterpart of the electron. It has the same mass and spin as an electron, but with a charge opposite in sign and equal in magnitude to that of the electron. If an electron and positron collide, they can annihilate each other and release their energy in the form of a photon. What are particle accelerators used for? Particle accelerators are used to generate beams of particles moving at very high speeds, which are typically then collided with matter or other particles to learn about funda- mental interactions. Most of the time the particles in question are subatomic particles, though atoms can also be used. Such experiments are used to address fundamental ques- tions in physics surrounding the structure of matter and space. Typical modern parti- cle accelerators are several kilometers long, with some operating in a linear fashion and 152 others in a large ring.

What is a quark? NUCLEAR CHEMISTRY Quarks are the fundamental particles that make up protons and neutrons, as well as several other types of particles. There are six types of quarks, which are referred to as dif- ferent “flavors.” These are named up, down, top, bottom, charm, and strange. Protons and neutrons are each made up of three quarks. Two up and one down quark make up a proton. Two down and one up quark make up a neutron. How do nuclei spontaneously decay? Nuclei can undergo several types of decay through spontaneous means without collid- ing or interacting with nuclei of other atoms. The most common types of nuclear decay are called alpha radiation, beta radiation, and gamma radiation. These differ by the type of fragmentation the nucleus undergoes during the decay process. What is alpha radiation? Alpha radiation involves the fragmentation of the nucleus into two particles, one con- sisting of two protons and two neutrons (an alpha particle, or in other words, a helium nucleus), and the other consisting of the remaining protons, neutrons, and electrons ini- tially present in the parent nucleus. Alpha decay decreases the number of protons in the nucleus by two and decreases the atomic mass of the nucleus by four amu. What is a beta particle? Beta particles are another type of particle that can be emitted during a nuclear decay process. A beta particle can be either an electron or a positron, which is the antiparti- cle of an electron. If it is an electron being emitted, one of the neutrons in the nucleus must become a proton to conserve charge in the process. Beta decay increases the num- ber of protons in the nucleus by one and leaves the atomic mass essentially unchanged. How is nuclear chemistry related to the alchemists’ goal of transmutation? Alchemists sought a way to turn common metals into gold, which we now know is not possible to do in any simple way. The reason is that transmutation would involve converting one element into another, which can’t be done by simple chem- ical processes. It would require a nuclear reaction to take place; either a heavy nu- cleus would have to divide into a gold nucleus and another byproduct, or two lighter nuclei would have to combine to form one of gold. Neither of these things happen readily. If early alchemists had recognized the distinction between more ordinary chemical reactions and the nuclear reaction they were looking for, it would likely have saved a lot of time and effort. 153

What is gamma radiation? While alpha and beta radiation is the loss of some particle from an atom, gamma radi- ation is the release of electromagnetic radiation (called gamma rays). This energy is typ- ically of a high frequency (Ͼ1019 Hz), which means it’s high energy (Ͼ100 keV) and can cause significant damage. Gamma radiation can easily penetrate deep into your body, un- like alpha and beta particles, causing damage to your cells and the DNA inside them. Sometimes this damage is useful, though, and some radiation therapies for cancer treat- ment make use of gamma radiation to kill the malignant cells. What holds nuclei together? The nucleus of an atom consists of neutrons, which are uncharged, and protons, which are positively charged. While the uncharged neutrons don’t feel an electrostatic attrac- tion or repulsion to other particles, the positively charged protons should repel each other. In fact, this repulsive force between the protons is quite strong because protons in the same nucleus are very close together. Thus the force that holds them together must be a very strong force. Indeed it is, and it’s even named the strong force. This strong force acts only over distances on the order of 10–15 m—a very very short distance. If the protons were to become separated by a more substantial distance, the strong force would decrease in magnitude faster than the repulsive force, and the protons would be pushed apart. It’s also often said that neutrons act as a sort of “glue” to help bind all of the neutrons and protons together, since there seem to be favored relationships between the number of neutrons and protons present in stable nuclei. Do all isotopes of an element decay at the same rate? No, actually each isotope decays at a unique rate. The most radioactive isotopes are those isotopes which decay most quickly. There are some elements (especially the heaviest ones) that don’t have any truly stable isotopes, and these can only be synthesized for fleeting amounts of time in laboratory settings. What is the half-life of a radioactive species? The half-life of a radioactive species is the amount of time it takes the quantity of the species to decrease by half. After one half-life, ½ of the initial quantity of material will remain, after two ¼ will remain, after three 1⁄8 will remain, and so on. Half-lives of ra- dioactive nuclei vary widely, and we’ll list just a few values below to give an idea of the range of timescales covered. Radioactive Nucleus Half-Life Carbon-14 5,730 years Lead-210 22.3 years Mercury-203 46.6 days Lead-214 26.8 minutes Nitrogen-16 7.13 seconds 154 Polonium-213 0.000305 seconds

What defines how long one second lasts? NUCLEAR CHEMISTRY One second is defined as 9,192,631,770 times the period of the electromagnetic radiation (see the “Physical and Theoretical Chemistry” chapter for more on electromagnetic radiation) corresponding to the difference in hyperfine energy levels in the ground state of a Cesium–133 atom. What does this mean? To begin, the difference in two closely spaced energy lev- els of a Cesium–133 atom defines a specific gap in energy. Using the relationship between the energy and frequency of a photon of light, this energy gap can be con- verted to a frequency of light. Recall that light is electromagnetic radiation. Also recall that the reciprocal of the frequency of light tells us the period of the oscil- lation of the electromagnetic fields that make up the light. The period tells us how long it takes the electric and magnetic fields to oscillate a single time, and one second is defined as 9,192,631,770 times this (extremely brief) time interval. As those 9,192,631,770 oscillations of the electric field of the light take place, the second hand of each clock on Earth moves 1/60th of a rotation forward. What is electron capture? Electron capture is a process that involves an electron combining with a proton to form a neutron. This decreases the atomic number of the element by 1 and leaves the atomic mass unchanged. Who was Marie Curie? Marie Curie was a famous French-Polish scientist, and she was the first person ever to be awarded two Nobel prizes, one in chemistry and the other in physics. She was also the first woman to ever win the Nobel Prize, and remains the only woman to have ever won two Nobel prizes in different fields. Curie was responsible for much of the pio- neering work in nuclear chemistry during the late nineteenth and early twentieth cen- turies. Much of her work focused on studying radioactive elements, and she discovered radium and polonium. Tragically, it was Curie’s work that also led to her death. During her career, the dangerous effects of radiation were not yet known, so she worked with- out the same safety precautions that would be taken today. Her death was the result of a condition known as aplastic anemia, brought on by her prolonged exposure to radia- tion in the laboratory. How is radiation exposure quantified? 155 The scientific unit for radiation exposure is the sievert (Sv), though several other types of units do exist. The maximum radiation exposure that is allowable for occupational ex- posure in the U.S. is 50 millisieverts (mSv). For comparison, the average natural back- ground level of exposure is roughly 3 mSv.

Pierre and Marie Curie working in their laboratory. Marie Curie was the first woman to ever win a Nobel Prize. In fact, she won two of the prestigious prizes. She studied, among other things, radioactive elements and discovered polonium and radium. N U C LEAR C H E M I STRY AT WO R K What is nuclear fusion? Nuclear fusion is the process by which two nuclei combine to form a single, heavier nu- cleus. Energy is usually released when two lightweight nuclei fuse, though for heavier nuclei, fusion generally requires an input of energy. Nuclear fusion can be used in bombs to cause a massive and rapid release of energy. Fusion is also responsible for the fact that stars burn bright and give off light and heat. What is cold fusion? A cold fusion reaction is one that takes place under ambient conditions using simple equipment. Such a fusion reaction would be extremely desirable, since it could allow for a simple and efficient means of energy production. What is nuclear fission? Nuclear fission is essentially the opposite of nuclear fusion. Here, a single nucleus di- 156 vides into two smaller nuclei. In the case of heavy atoms, this is often accompanied by

Is cold fusion really possible? NUCLEAR CHEMISTRY In the late 1980s, reports surfaced of experimentally realized cold fusion, excit- ing the scientific community. It turned out, however, that these reports were false, and nobody was able to reproduce the results of what were initially reported as relatively simple experiments. Since these experiments were disproved, other credible reports of cold fusion experiments have indeed surfaced, and thus it does appear that cold fusion is possible in principle. Unfortunately, the energy released from the few successful experiments has been much smaller than the amount of energy needed to actually run the experiments, making the feasibility of cold fu- sion as a source of energy production unlikely. Compared to the initial burst of in- terest, mainstream scientists have generally lost interest in the topic, though there remains a group of fringe experimentalists who still seek to make cold fusion for energy production a reality. If such experiments could work, they would certainly be of great interest to the scientific community, but today most believe that it just isn’t possible to generate enough energy from cold fusion sources to make it a vi- able source of energy production. the release of heat. For example, the radioactive decay of uranium-235 can be used to generate the heat used to drive turbines to generate electricity in nuclear power plants. The use of nuclear fission to harness energy for use by humans is typically considered the much more viable choice (as opposed to nuclear fusion). Is mass conserved during a fission process? Almost, but not quite. A small amount of mass is given off in the form of energy. Specif- ically, the relationship between the amount of energy, E, released and the amount of mass, m, that becomes energy is given by the famous relationship E ϭ mc2, where c is the speed of light. How can radioactivity be measured? 157 Radioactivity is measured by detecting the products of radioactive decay processes. The most well-known instrument used for this purpose is the Geiger counter. A Geiger counter is sensitive to the products of nuclear decay, including alpha and beta parti- cles and gamma rays. The units used to quantify radiation are the Curie or the Bec- querel, which describe the number of nuclear decays a substance undergoes per unit of time. In many cases, it may not be necessary to directly detect the radiation being given off at this instant, but rather to just determine the isotopic ratio of an element present in a sample. This can be done using techniques borrowed from analytical chemistry, such as mass spectrometry. Information on the isotopic ratio present, along with knowl-

A Geiger counter is a useful tool for measuring radioactivity in almost anything. It can detect alpha and beta par- ticles, as well as gamma rays. edge of the half-life of the isotope in question, can be related to the age of the sample being studied. How does radioactive dating work? Radioactive dating (also called radiometric dating) is a technique used to determine the age of a sample based on the ratio of isotopes of an element present in the sample. Using the known half-life of the isotope being studied, along with knowledge of the natural abundance of the isotopes present at the time the sample was formed, the age of the sample can be determined. To obtain an accurate age for a sample, it is required that none of the isotopes being measured have been able to escape or re-enter the sample over the course of its lifetime. Otherwise this could serve to establish a ratio of isotopic abundances that is not representative of that based purely on the half-life of the isotope whose decay is being measured. What is a nuclear chain reaction? A nuclear chain reaction is a string of reactions that occurs when a given nuclear reac- tion causes, on average, at least one more nuclear reaction to take place. Such chain re- actions are important for the generation of nuclear power and also for nuclear weapons. Uranium-235 is responsible for the chain reaction that generates power in nuclear reac- tors and in some bombs as well. Uranium-238 is the more common isotope, so it is typ- ically necessary to first enrich the uranium to be used in the 235 isotope. When a neutron 158 collides with uranium-235 it generates uranium-236, which then undergoes fission to re-

lease energy and further neutrons that can NUCLEAR CHEMISTRY collide with other uranium-235 atoms, causing the chain reaction to continue. How does an atomic bomb work? Uranium-235 chain reactions start because the ra- dioactive substance naturally emits neutrons that Atomic bombs (A-bombs) are based on nu- then collide with other atoms. In a nuclear bomb, clear chain reactions that occur very the goal is to let the reaction reach a critical point rapidly, causing a huge release of energy where there is an explosion, but with a nuclear reac- in a very short amount of time. In early de- tor the tricky part is controlling the reaction. signs, two pieces of uranium would be fired at one another in the core of the bomb, initiating the fission chain reaction responsible for the explosion of the bomb. As the bomb starts to detonate the core of the bomb expands, and it is necessary that pressure be applied against the expanding core while the fission process takes place. Within a fraction of a second after detona- tion, the explosion takes place. These are the type of bombs that were used at Hi- roshima and Nagasaki in World War II and are the only nuclear weapons that have been used in war to this day. What’s the difference between an H-bomb and an A-bomb? The hydrogen bomb (H-bomb) is actually significantly more destructive than even an A- bomb. While A-bombs release energy via chain fission reactions (breaking apart heavy nuclei), H-bombs release energy through fusion of light nuclei. This energy comes from an overall increase in stability due to the strong force that holds nuclei together as the light nuclei fuse to create heavier ones. To give an idea of the relative powers of these two weapons of mass destruction, consider that the A-bomb dropped on Hiroshima had a force on the order of 10 kilotons (explosive force equivalent to 10,000 tons of TNT), while a common H-bomb has a force on the order of 10 megatons, or 1,000 times the explosive force of the A-bomb used at Hiroshima. How is radiation used in medicine? 159 We should begin by pointing out the distinction between radiation used in nuclear med- icine/radiopharmaceuticals (more akin to the other topics of this chapter) and electro- magnetic radiation (light of different wavelengths). Nuclear medicine is the branch of medicine most closely tied to the concepts of nuclear chemistry discussed in this chapter. Diagnosis via nuclear medicine typically involves the injection of a radiopharmaceutical into the body, and the radiation released by this drug can

then be monitored to gain information about organ function, blood flow, the loca- tion of a tumor, or to locate a fractured bone. In some cases, the use of nuclear med- icine can allow for earlier diagnosis than with other imaging techniques. In terms of using electromagnetic ra- diation for medical applications, perhaps one of the first treatments that come to mind is radiation therapy, which is used to fight against a broad range of cancers. This involves using focused electromagnetic ra- diation to damage the DNA in the tissue of An atomic bomb blast (illustration shown here) re- a tumor while hopefully not causing too leases huge amounts of energy by creating a fision much damage to the surrounding healthy chain reaction within the bomb. tissue. The goal is to damage the DNA of cancerous cells so that they are unable to reproduce, hopefully killing the tumor with time. Beams of radiation are focused onto the tumor from different angles to minimize the effect on any one area of healthy tissue. X-rays and CT scans are two commonly used, noninvasive medical techniques that make use of electromagnetic radiation to take pictures of what’s going on inside the human body. It should be noted that prolonged exposure to the X-rays used in these procedures can be harmful and are capable of causing cancer themselves over long pe- riods of time. How are isotopes made? Specific isotopes of an element can be obtained in one of two ways: either by separation of the desired isotope from a naturally occurring sample or by synthesis of the desired isotope. Since the different isotopes of an element all have the same chemical properties, they can be quite difficult to separate. The separation techniques used to separate dif- ferent isotopes are thus based on their differences in mass, rather than on differences in chemical properties. Some of the methods used include separation by diffusion in the gas or liquid phases, centrifugation, ionization and mass spectrometry, or chemical methods based on differences in reaction rates due to different atomic masses. Different isotopes of an element can also be generated synthetically. One way to do this is to fire high-energy particles at the nucleus of an atom. Depending on the situa- tion, this can either cause a particle to be emitted from the parent nucleus (generating a lighter nucleus) or the fired particle can be absorbed (generating a heavier nucleus). It is also possible to synthesize isotopes of some elements by making use of another nat- urally occurring nuclear reaction, such as when the particles released by one nuclear fis- 160 sion reaction are absorbed by another nucleus.

Which elements are man-made? NUCLEAR CHEMISTRY Actually, we can make a lot of elements synthetically. Here’s a list of all of the man-made 161 elements: technetium (Tc)—43 (the first man-made element) promethium (Pm)—61 neptunium (Np)—93 plutonium (Pu)—94 americium (Am)—95 curium (Cm)—96 berkelium (Bk)—97 californium (Cf)—98 einsteinium (Es)—99 fermium (Fm)—100 mendelevium (Md)—101 nobelium (No)—102 lawrencium (Lr)—103 rutherfordium (Rf)—104 dubnium (Db)—105 seaborgium (Sg)—106 bohrium (Bh)—107 hassium (Hs)—108 meitnerium (Mt)—109 darmstadtium (Ds)—110 roentgenium (Rg)—111 copernicium (Cn)—112 ununtrium (Uut)—113 ununquadium (Uuq)—114 ununpentium (Uup)—115 ununhexium (Uuh)—116 ununseptium (Uus)—117 ununoctium (Uuo)—118 How do nuclear power reactors work? A nuclear power reactor works by generating heat from a controlled fission reaction, which then generates steam used to drive turbines to generate electricity. The fuel for the nuclear reactor is typically uranium-235 or plutonium–239. What is a thorium reactor? A thorium fuel cycle is also possible for use in nuclear power reactors. This involves using thorium–232 to generate uranium-233, which is capable of undergoing fission processes to generate energy in the form of heat.

Nuclear reactors work by generating heat from controlled fission reactions. Breeder reactors actually create more fissionable material than they use and are self-sustaining. What is a breeder reactor? A breeder reactor is a type of nuclear reactor that is capable of generating fissile mate- rial (material that can sustain a chain fission reaction) faster than it uses it up. This is accomplished by using the neutrons given off in the fission reaction to generate addi- tional isotopes capable of fusion. Typically this involves the use of either thorium to generate fissile uranium or uranium to generate fissile plutonium. What is radon? Radon is an element that is widely known for its potential to cause cancer. It is the heaviest gas known to man, with a density roughly nine times greater than that of air. It is usually found in soil and rocks, though it can also be found in water. Fortunately, radon detectors are commonly available that allow you to test your home for elevated radon levels. What are some of the worst nuclear disasters in history? A few of the worst nuclear disasters in history are those which took place at Three Mile Island in the USA in 1979, at Chernobyl in the Ukraine in 1986, and more recently fol- lowing an earthquake in Fukushima, Japan, in 2011. Nuclear disasters are very danger- ous if they do occur, and the possibility of a nuclear disaster represents a primary reason that some people oppose the construction of new nuclear power plants. 162

POLYMER CHEMISTRY P O LYM E R S AR E M O LE C U LE S TO O! What is a polymer? Polymers are large molecules, usually made up of smaller repeating units. The word it- self, polymer, means “many parts” in Greek. You probably started thinking about plas- tics (like milk jugs and plastic cups) when you read the title of this chapter. Plastics are common examples, but polymers also play important roles in all plants and animals, including you. What is a monomer? If polymer means “many parts,” a monomer is “one part” of that whole. A monomer is a molecule that is attached to many copies of itself to make a polymer molecule. Usu- ally these bonds are covalent, but not always. How are polymers different than small molecules? 163 So many ways! Polymer chemistry and polymer physics are big areas of research both in the recent past and today because connecting a bunch of small molecules into one big one results in lots of interesting changes. To give you a metaphor, let’s talk about pasta. Start with uncooked macaroni and un- cooked spaghetti: If you try to move your hand through a bowl of uncooked macaroni you won’t have much trouble, but if you had spaghetti noodles all lined up and you tried to move your hand through them (in either direction!), you’ll run into problems. You either need to break the noodles or you need to carefully thread the noodles through your fingers. Both of these actions require energy (enthalpy in the first case and en- tropy in the second).

Now let’s cook those noodles. Stick a fork in each of the bowls and spin it around. With macaroni, nothing happens, but the spaghetti starts to wind around your fork, gets tangled up, and so on. Macaroni, a collection of small molecules…I mean, noodles, is totally different than polymers (spaghetti) which are also made up of flour and water, but are much longer. The raw and cooked spaghetti aren’t just easy to imagine, they’re great ways to think about polymers in different states (solids and liquids, glassy states and polymer melts). Are all polymer chains the same size? No. Let’s stick with the macaroni metaphor to understand this. Imagine you’re string- ing noodles together to make a macaroni necklace. You can put as many noodles on a single string as you want. If you have two strings, you can put an equal number of noo- dles on each string or make one longer than another. Again, the macaroni noodles are monomers, which form polymers when we string them together. So if all polymers are not the same size, what is the weight of a polymer? Good question. If we know the number of monomers that make up a polymer chain (technical term: degree of polymerization), then the molecular weight of the polymer is the molecular weight of the monomer multiplied by that number of monomers. How do you measure molecular weight of a polymer? The most common way is based on size. The technique is known as size exclusion chro- matography, or gel permeation chromatography. The sample is passed through a col- umn that has a porous solid material. The smaller polymers can work their way into those pores, while larger molecules don’t interact with the solid material. The biggest molecules, because they don’t interact with the solid phase, come out of the column first followed by smaller and smaller molecules. The time it takes for a polymer to get through the porous column is related to its molecular weight (okay, technically it’s based on the hydrodynamic volume, but let’s let this approximation slide). In practice, these instru- ments are calibrated using standard polymer samples of a known molecular weight. What is molecular weight distribution? We just talked about the fact that polymers can have different molecular weights. Of- tentimes in reactions that make polymers a range of molecular weights are produced. The molecules may be composed of the same repeating unit (monomer), but for a num- ber of reasons the chains are different in length. It turns out that this distribution of lengths is important to a number of polymer properties. The details of how this num- ber is calculated are not worth going into; it’s sufficient to know that a higher molecu- lar weight distribution means that there is a larger spread of polymer chain lengths. A distribution of 1.0 would mean that every single polymer chain has the exact same mol- 164 ecular weight.

Does polymer stereochemistry matter? POLYMER CHEMISTRY In lots of examples, it does. Let’s stick with polypropylene for now. Isotactic polypropylene is a crystalline material with a melting point around 160 °C. The crystallinity is due to the perfect arrangement of the methyl groups along the poly- mer backbone. This crystallinity makes the material very tough, so it’s used in all sorts of applications—from pipes to plastic chairs and carpeting. But if there are errors along the polymer chain (methyl groups pointing in the wrong direction) the melting point decreases, and the plastic loses its strength. Do polymers have stereochemistry like small molecules? Yes! The most common example is polypropylene. This polymer has a methyl group at- tached to the backbone of the polymer. If the methyl groups are all on the same side of the chain, the stereochemistry is known as isotactic (top structure below). If the arrange- ment of the methyl groups alternates which side of the chain it’s on, the polymer is called syndiotactic (bottom structure below). If there’s no order at all to the substituents, we call the polymer atactic. Are all polymers linear chains? No, and this is another way that chemists classify these really big molecules. The major types of polymer shapes (technical term: topology) are linear, branched, and crosslinked networks. Linear polymers are chains of monomers joined together, like a noodle or a rope. If there is a point along a polymer chain where a second chain starts, like a fork in the road, this arrangement is referred to as branched. What is a crosslinked polymer? 165 When a bond is formed between two polymer chains (and technically not at the chain ends), the product is called a crosslinked polymer. Creating linkages between chains usually increases their viscosity (so more like molasses than olive oil) and creates elas- tic properties like those found in rubber bands. At higher levels of crosslinking, polymers can even become stiff or glassy.

P O LYM E R S I N AN D ARO U N D YO U What polymers are found in nature? There are a ton! Proteins, enzymes, cellulose, starch, and silk are all polymers. Is DNA a polymer? It is. DNA contains two long polymers of sugars (called nucleotides). Attached to each sugar molecule are phosphate groups and a nitrogen base (technically a nucleobase). The sequence of these nucleobases encodes the information in DNA. (For more on DNA, se the “Biochemistry” chapter.) What is cellulose? Cellulose (see diagram below) is linear polysaccharide—“poly” for many and “saccha- ride” means sugar, so cellulose is a chain of sugar molecules. It’s an amazing molecule: it’s the most abundant organic compound on the planet because it is the main compo- nent in plant cell walls. Cellulose is highly crystalline because of the way the sugar mol- ecules are connected and because of the fact that it’s made up of a single enantiomer of glucose. Like polypropylene, highly crystalline polymers like cellulose are very strong— strong enough to make trees stand up straight. Is starch different than cellulose? Starch is also a polysaccharide, like cellulose, but it is much less crystalline. The major component of starch is amylopectin, which is a highly branched polymer, while cellu- lose is strictly linear. These branches prevent amylopectin from crystallizing as well as cellulose. Starch is an excellent source of energy (or stored sugar) for plants and animals for these reasons: Because it is less crystalline than cellulose it is more soluble than cel- lulose, and the branched structure also means there are more end groups at which en- zymes can start “chewing” the polymer apart. What is rayon? You probably know what rayon fabric looks or feels like—the best Hawaiian shirts (and 166 much of 1980s fashion) were made of it. What’s fascinating about rayon is that it is not

really a synthetic or a natural fiber. Rayon is a chemically modified cellulose polymer, POLYMER CHEMISTRY first prepared in the 1850s. While there have been a number of ways of preparing this “artificial silk,” the Viscose method led to the first commercial production of rayon. This method treated cellulose with a combination of sodium hydroxide and carbon disulfide as indicated below. How was rayon discovered? 167 The first artificial silk was probably prepared by a Swiss chemist, Georges Audemars, in 1855. Audemars mixed the pulp of mulberry bark (chosen likely because silkworms eat mulberry leaves) and a rubber gum and used a needle to pull out long fibers of mater- ial. This was a rather labor-intensive and difficult process and could not be done in any economic way. Some accounts also claim that Audemars drew fibers of nitrocellulose (the product of mixing nitric acid with cellulose); in addition to being a delicate process, the resulting fibers of nitrocellulose were highly flammable. Hilaire de Chardonnet, a French engineer, was another key player in the history of artificial silk. Working with Louis Pasteur in the 1870s, the legend claims that he spilled a bottle of nitrocellulose while working in a photography darkroom. The spilled solution was left to evaporate, and Chardonnet returned later to clean up his mess. Wiping up the residue, he noticed long, thin fibers had formed. Chardonnet received a patent on this material, but again the flammability kept it from achieving large market adoption. The Viscose method mentioned earlier was finally worked out in 1894 by English chemists Charles Frederick Cross, Edward John Bevan, and Clayton Beadle. This method was a commercial success, and the fabric was manufactured first by Courtaulds Fibers in the United Kingdom and then Avtex Fibers in the United States. Where does rubber come from? Rubber trees! No kidding. Natural rubber is collected from rubber trees like maple syrup comes from maple trees, except the syrup is latex sap. It’s a polymer of isoprene where each carbon–carbon double bond along the chain has cis-stereochemistry. Although there are man-made alternatives produced artificially, even today about half of the rub- ber produced each year on our planet does come from rubber trees.

What is vulcanization? Natural rubber that comes directly from rubber trees looks nothing like your car or bike tires. It’s sticky, doesn’t hold its shape when it gets warm, and if you live where it snows, it can get brittle. That sounds like an awful material to make a tire out of! Vulcanization creates crosslinks in the rubber with the addition of sulfur to the natural rubber chains and improves all of these properties. What is an addition polymerization reaction? The easiest way to describe an addition polymerization reaction is that monomers are bonded together without the loss of any atoms of the monomer. There are more com- plicated ways to classify this type of reaction based on kinetics, but it essentially boils down to this fact. How is a condensation polymerization reaction different? Addition polymerization reactions do not involve the loss of any atoms from the monomer, but condensation polymerization reactions do. The molecule that is lost is al- 168 most always water.

What do the recycle numbers mean on plastic bottles? POLYMER CHEMISTRY These are technically called Resin Identification Codes (RSI) and were intro- duced in the 1980s to make it easy to separate plastics for recycling. The num- bers correspond to what kind of polymer the item is made of and have no other meaning. They are not in any sort of order based on how easy or hard it is to re- cycle the resins, despite rumors you might have heard about this. RSI Plastic 1 Polyethylene terephthalate 2 High-density polyethylene 3 Polyvinylchloride 4 Low-density polyethylene 5 Polypropylene 6 Polystyrene 7 All others What is a thermoplastic? If a polymer becomes soft when it is heated (and then hardens again when cooled), it’s known as a thermoplastic. The temperature at which the polymer softens depends on what it’s made of and on the size of the polymer chains. Thermoplastics are easy to recycle because they can be remolded when they’re hot. What is a thermoset? Unlike a thermoplastic, thermoset materi- als are cured such that they don’t soften when exposed to heat (at least up to a cer- tain point). This curing step usually intro- duces a lot of crosslinks between polymer chains, which creates a rigid network. These materials are much more difficult to recycle, but are used where high-tempera- ture stability is needed. What is PET (polyethylene According to the EPA, in 2010 Americans only recy- 169 cled eight percent of their plastic waste. We can do terephthalate)? better. About seventy-five percent of the packaging we use is recyclable. Polyethylene terephthalate is a thermo- plastic that is used in both textiles, where it’s called polyester, and in bottling, where

the same polymer is called PET. PET is an alternating polymer of ethylene and tereph- thalate monomers. This material is really good at preventing gases from diffusing through it, so it’s great for keeping carbonated drinks fizzy. What is HDPE (high-density polyethylene)? High-density polyethylene is technically any polyethylene with a density of 0.93 to 0.97 g/cm3. The density in polyethylene is controlled mostly by the number of branch points along the polymer chain. HDPE has very few branches, so the chains can stack together very closely. This tight packing makes it a very strong polymer, so it’s used to make things like bottlecaps, milk jugs, and Hula Hoops. What is LDPE (low-density polyethylene)? If the density of a polyethylene is between 0.91 to 0.94 g/cm3 (yes, there’s a little over- lap in these ranges) it’s called low-density polyethylene. To get to this density, the poly- ethylene chains have more branching than in HDPE but still only a few percent of the atoms along an entire chain. These branches prevent the chains from stacking together quite as well, which makes the material softer and more flexible. With those properties, LDPE finds use as trash, grocery, and sandwich bags, and that “clingy” food wrap (al- though the original Saran® Wrap was not LDPE—see below). 170

What is PVC (polyvinyl chloride)? POLYMER CHEMISTRY If one of the hydrogen atoms on every ethylene monomer in polyethylene is replaced by a chlorine atom (note that this is not how this material is actually made!), you get PVC, or polyvinylchloride. It’s the third-largest-volume polymer produced each year behind polyethylene and polypropylene. It is a very tough polymer, so it is used to make pipes and flooring among many other things. PVC can also be softened (technical term: plas- ticized) by introducing small organic molecules, like phthalates (a benzene ring with two esters). Among other applications, plasticized PVC is used to insulate electric wires and to make your garden hose. What is my credit card made of? Also PVC—but for the material in credit cards, no plasticizer is added. To manufacture a credit card, usually a few thin sheets of PVC are glued together. What is PP (polypropylene)? If instead of substituting a chlorine atom we add a methyl group to each ethylene monomer, we get polypropylene. Recall from earlier that this introduces stereochemistry along the polymer. We’ve already mentioned that the arrangement of the methyl groups along the polymer chain can have large effects on the melting point and other physical properties. You can likely find polypropylene all over your house from dishwasher-safe food containers to synthetic carpets (especially outdoor carpeting) and an increasing weight fraction of your car, including the bumper and the casing for the battery. It can also be made into ropes, which are quite strong and resistant to weather, so they are frequently used in fishing and farming. Polypropylene is also used for many medical applications be- cause it is capable of withstanding the high temperatures required to sterilize. What is PS (polystyrene)? 171 If one of the hydrogen atoms on every ethylene monomer in polyethylene is replaced by a benzene ring (note again that this is not how this material is actually made!), you get PS, or polystyrene. Polystyrene is usually the fourth-largest-volume polymer produced globally, with billions of pounds made annually. Polystyrene can be manufactured into

parts (like CD cases, furniture, and eating utensils), or air can be mixed with the polymer to make a foam used in insulation both for your house and your coffee cup. Styrofoam® is a trademarked name held by the Dow Chemical Company for foamed polystyrene. What is Saran® Wrap? Saran® Wrap is a trade name (another held by the Dow Chemical Company) for polyvinylidene chloride. If two of the hydrogen atoms on every other carbon in poly- ethylene are replaced by chlorine atoms (note yet again that this is not how this mate- rial is actually made!), you get PVDC, or polyvinylidene chloride. It was discovered by accident in 1933 by Ralph Wiley, who was having trouble washing this strange mater- ial out of the bottom of a piece of glassware. The actual polymer they were trying to make was poly(perchloroethylene)—where every hydrogen is replaced by a chlorine atom. It was just before WWII that a breakthrough was made that allowed the scientists to make film from this new material. It was quickly adopted by the Army to wrap equip- ment being transported by sea in order to prevent corrosion from saltwater and other applications to keep soldiers dry in jungle environments. After WWII, Dow found a new use for the material and introduced a PVDC film product for wrapping food called Saran® Wrap. The clingy food wrap you buy today is not PVDC, however. This material was phased out due to environmental and health concerns of those chlorine atoms, and low- density polyethylene took its place. Okay, but why was PVDC ever called Saran® Wrap? Many industrial trade names have no interesting story behind the creation. Saran® Wrap is an exception. You might think that Ralph Wiley was responsible for naming this ma- terial, having discovered it. Nope. Ralph Wiley’s boss, John Reilly, named the material after his wife and daughter—Sarah and Ann. What is nylon? Nylon is a synthetic polymer formed by the condensation of a dicarboxylic acid with a 172 diamine. This reaction forms an amide bond and releases a molecule of water. The term

What makes my cooking pans “nonstick”? POLYMER CHEMISTRY The coating that is placed on cookware is usually polytetrafluoroethylene (PTFE), marketed as Teflon® by DuPont®. The strength of the C–F bond, and its reluctance to interact with just about anything else, makes Teflon® very heat- resistant and slippery stuff. Aside from coating cookware, it’s used to make gears and bearings, and it’s a key component of Gore-Tex (the material your waterproof jacket is made of). “nylon” is a generic name for these types of polymers, but one of the common nylons is called “Nylon 66.” The numbers signify the number of carbon atoms in the amine (6) and the acid (6) reactants. When was nylon discovered? Nylon 66 was first made by Wallace Carothers, a scientist working at DuPont®, on Feb- ruary 28, 1935. Dr. Carothers also contributed to the discovery of neoprene, which is used to make suits used for scuba diving. What was nylon first used for? The first commercial application was probably toothbrush bristles. For centuries tooth- brushes were made of coarse animal hairs (usually boar) until Dupont® introduced Doc- tor West’s Miracle Toothbrush in 1938. What is silicone? Is that the same as silicon? Silicon is an element, while silicone is a polymer with a backbone of silicon and oxygen atoms. These polymers are very resistant to heat and have a rubbery feel. The latest squishy cookware and bakeware is made out of silicone. What is glue? 173 There are many different types of adhesives, but you’re probably thinking of that white glue you had as a kid. This type of glue is known as a “drying adhesive” because it hard- ens by evaporation of a solvent. In the case of white glue, the solvent is water and the sticky stuff that gets left behind is polyvinyl acetate.

Does hairspray contain polymers? Yep—and almost the same ones that make white glue and acrylic paints. Many hair- sprays contain vinyl acetate (or something similar), polyvinyl pyrrolidone, and/or lots of other variants. Like glue, you also need a liquid to disperse or dissolve the polymers, but in hairspray a mixture of alcohol and water is normally used. What’s in paint? There are three main ingredients in paint: binder, solvent, and pigment. Binder is the stuff in paint that forms an adhesive so that it sticks to the wall. Unlike glue, it’s not a polymer in paint (at least not a fully formed one), but instead monomers or short poly- mer chains that react (crosslink) to form larger polymer networks as the solvent evap- orates. The solvent in paint is there to make the paint solution the right thickness so that you can easily put it on the wall without it dripping all over the floor. Then the solvent evaporates, driving the formation of the crosslinked polymer networks. Pigments are, of course, used in paint so that not everything is painted white (though white paint still needs pigments to be white!). How is recycled plastic used to make fleece? Fleece can be made from polyethylene terephthalate (PET) bottles. The first step is to wash and then mechanically crush the bottles, shaping the plastic into small chips. The chips can then be heated and forced through tiny holes in a metal plate (called a spin- neret), which forms fibers that harden as they cool to room temperature. These fibers are wound onto a spool as they are formed, and they can subsequently be stretched to improve their strength. Machines can then be used to texturize and cut the fibers to their desired length and be used to make fleece cloth for clothing, blankets, etc. There are polymers in my shampoo and conditioner, too? There are! Many ingredients in shampoos and conditioners are similar to most soaps 174 (surfactants, etc.), but cationic polymers play important roles in these products. One

family of these polymers is named “polyquaterniums,” which is closer to a trade name POLYMER CHEMISTRY than a technical chemistry name (the structure of one member of this family, “polyquaternium 1,” is shown below). All of these polymers contain positive charges, which allows them to form ionic bonds with hair strands. This prevents the polymers from being rinsed away with water. Once coated, your hair strands are now less likely to stick to adjacent strands and appear shinier. What is fiberglass? Fiberglass is a polymer made from a plastic matrix that is reinforced by fibers of glass. It is a popular material because it is inexpensive to make, and its strength and weight properties compare favorably against those of metals for many applications. Fiberglass has a wide range of applications and is commonly used in glider aircraft, boats, cars, showers and bathtubs, roofs, pipes, and surfboards. What absorbs liquid in diapers? The general term for the materials that absorb water in diapers is “super-absorbent polymers”; these same compounds are used to chill drinks and to make fire retardants and fake snow. Modern absorbing materials are typically sodium salts of polyacrylic acids, and these can become almost entirely water by weight and as much as 30–60% by volume. Wait—absorbent polymers are also used to chill drinks? How does that work? If you stuff a cup holder with absorbent polymer and then soak it in water, the polymer will obviously swell up. The water will slowly evaporate out of the polymer, which re- duces the temperature of the polymer gel and ultimately your drink. What is Styrofoam®? 175 Styrofoam® is a brand name (owned by the Dow Chemical Company) for expanded poly- styrene foam. It’s 98% air, which is why those styrofoam® coffee cups are so light (and actually buoyant). In addition to disposable food containers, polystyrene foam is used in building and pipe insulation, packing peanuts, and that green stuff they use for holding together fake flower arrangements.

And spandex? What’s that? Spandex (as it’s called in North America; Europe knows the same material as “elastane” and the Brits refer to it as “lycra,” but that’s confusing because Lycra® is actually a trade name) is a rigid copolymer of polyurethane and polyurea and is a rubbery material, like polypropylene oxide. These two polymers do not mix well, so tiny domains of each poly- mer form. It’s this separation (of the hard bits from the soft bits) that gives Spandex its stretchy, yet strong, behavior. 176

ENERGY ENERGY SOURCES What is the breakdown for fuel source usage globally? Roughly 32.4% of the world’s energy is used in the form of oil. About another 27% comes from coal sources, and another 21% from natural gas. Together, these three sources ac- count for a total of over 80% of the energy sources used globally. The remaining 20% comes primarily from combustible renewables and waste (10%), nuclear energy (6%), and hydroelectric sources (2%). As you can see, some of the commonly discussed green energy sources, like solar energy or energy collected by wind turbines, do not yet make signifi- cant contributions to the breakdown of energy sources commonly used on a global scale. Energy Sources Worldwide* Energy Source % of Total Used oil 32.4 coal/peat 27.3 natural gas 21.4 biofuels and waste 10.0 nuclear 5.7 hydro 2.3 other 0.9 *According to a 2010 survey published by the International Energy Agency (http://www.iea.org/publications/freepublications/publication/kwes.pdf). What is crude oil? 177 Crude oil is the oil that is found underground in natural reserves. It is formed from the natural decay of living things that were in the sea millions of years ago, which is why it’s

commonly called a “fossil fuel.” The key in- gredient that makes oil a valuable energy source is hydrocarbons. As we discussed briefly in “Chemical Reactions,” hydrocar- bons release substantial amounts of energy when they undergo combustion reactions. How is oil refined? Oil refineries like this one can either refine oil through distillation processes or through chemical There are several processes that can be used methods. to refine oil. The oldest way is through dis- tillation, which involves heating up the crude oil slowly, allowing the different hy- drocarbons to boil off one at a time, and then collecting them as vapors. There are also newer chemical refinement methods that can use a chemical reaction to convert one type of hydrocarbon into another. What is cracking? Fluid catalytic cracking is the name for the process that converts high-boiling hydro- carbons found in crude oil into lighter hydrocarbons that are more useful as gasoline and other products. These units operate continuously for years at a time, and there are hundreds of them around the world. In addition to the high temperature and heat that these reactors use to break down long hydrocarbons, there are catalysts used to help speed up the process. These catalysts are usually strong acids in the form of zeolites, specifically faujasite, which is a mixture of silica and alumina. What is fracking? Hydraulic fracturing, or “fracking,” is a technique used to break up rock deep under- ground in order to release the natural gas trapped inside it. The fluid injected into the rock is generally water-based, but the specific mixtures of chemicals dissolved in that water are generally a closely guarded secret. Using fluids to crack rocks is an old tech- nique dating back over a hundred years, but modern techniques trace back to experi- ments in the 1940s by Floyd Farris and J.B. Clark, who worked for Stanolind Oil and Gas. Where does coal come from? Coal is formed from trees and plants that died hundreds of millions of years ago. The dead plant material eventually became buried deep underground, which placed a large amount of pressure on it. Over time the plant material became compressed and hard- ened, eventually forming coal. While this resource can be replaced over very long peri- 178 ods of time, we use it much, much faster than it can ever be regenerated.

If fracking is an old technique, why does it seem to be taking off now? ENERGY The second key innovation that has allowed fracking to grow so fast in recent years is the ability to drill wells horizontally. Technically any well that isn’t ver- tical is called directional or slant drilling. Hydraulic fracturing can release trapped natural gas from a much wider area this way, which has led to the recent surge in this technique. How does a nuclear reactor generate energy? Nuclear power comes from the energy released in fission processes, like the ones we discussed in “Nuclear Chemistry.” The nuclear reactors used for commercial energy production make use of controlled chain reactions of uranium-235. Upon colliding with a neutron, uranium-235 decays spontaneously into two lighter atoms, releasing energy for each atom of uranium that is split. The amount of energy released in the fission process is very large compared to that released from the combustion reactions that we use to obtain energy from fossil fuels. The result is that we can get a lot of energy from a relatively small amount of uranium. In fact, a kilogram of enriched uranium can gen- erate about the same amount of energy as eight million liters of gasoline! Where do we get uranium? Uranium is mined largely in Kazakhstan, Australia, and Canada, although the U.S., South Africa, Namibia, Niger, Brazil, and Russia are also significant producers. Uranium is mined using a leaching process that allows it to be dissolved from ore deep in the mine and then pumped to the surface, where it is extracted and concentrated for use. POLLUTION What are the potential dangers of nuclear energy? 179 The main risks associated with nuclear energy have to do with health hazards of radia- tion poisoning. This isn’t the same type of radiation we’re talking about with light and microwaves (that type is electromagnetic radiation). The radiation associated with nu- clear energy involves subatomic particles, like neutrons, given off from nuclear decay processes. Exposure to this type of radiation can cause cancer or genetic defects, among other problems. Since the fission processes used to generate energy are normally well controlled, though, nuclear energy is generally one of the cleanest energy sources avail- able. Nuclear reactors only become dangerous in the event of a meltdown or other dis- aster, but the chances of these happening are extremely small. Modern nuclear power

plants are built with a series of fail-safes such that a series of multiple, extremely unlikely events would have to happen, one after another, for a meltdown to occur. What pollutants do nuclear power plants generate? Not many, unless something goes wrong. No greenhouse gases (like carbon dioxide) are generated in nuclear power plants, and each only produces roughly one cubic meter of waste per year. As long as this is properly taken care of, nuclear power plants are one of the cleanest forms of energy. What pollutants do coal power plants generate? Burning coal introduces pollutants such as nitrogen oxides, sulfur dioxide, and mer- cury into the atmosphere. A large amount of carbon dioxide, the principal pollutant im- plicated in global warming, is also produced by coal power plants. Aside from global warming, many of the pollutants from coal power plants are bad for human health and also cause smog, soot, and acid rain. The solid waste produced by burning coal can also be harmful to the environment. The ash from coal power plants is typically composed of about 5% pollutants or dangerous substances such as arsenic, cadmium, chromium, lead, and mercury. This can also cause pollution in water if it is not disposed of prop- erly, and there have been many cases of water contamination caused by improper dis- posal of this ash. Which energy sources produce the most CO2? By far, it’s the fossil fuels: oil, gas, and coal. It’s the CO2 from all those combustion re- actions that makes up about 96.5% of all the carbon dioxide emissions. Which countries produce the most CO2? Based on data for the year 2009, the top twenty-five, in order, are: 1. China 14. Brazil 2. United States 15. Australia 3. India 16. Indonesia 4. Russia 17. Italy 5. Japan 18. France 6. Germany 19. Spain 7. Canada 20. Taiwan 8. South Korea 21. Poland 9. Iran 22. Ukraine 10. United Kingdom 23. Thailand 11. Saudi Arabia 24. Turkey 12. South Africa 25. Netherlands 180 13. Mexico

How much CO2 production comes from automobiles? ENERGY According to the Environmental Protection Agency (EPA), the transportation industry as a whole accounts for 33% of all CO2 emissions in the U.S. Auto- mobiles used for personal transportation account for 60% of this, or 20% of total U.S. CO2 emissions. The remainder comes from other sources of transportation, such as large diesel vehicles and airplanes burning jet fuel. What is carbon sequestration? Carbon sequestration and carbon capture are processes to remove carbon dioxide (CO2) from the atmosphere and store it. This is exactly what plants do when they convert CO2 into other molecules like sugars and proteins. Carbon dioxide can also react with water and limestone (CaCO3) to form calcium bicarbonate (Ca(HCO3)2) in an inorganic ex- ample of natural carbon sequestration. Generally, though, this term is used today to refer to man-made processes for removing CO2 from the atmosphere or for capturing it before it gets released (like from power plants). A number of approaches to long-term carbon dioxide storage are being, or have been, tried, including pumping the gas deep underground into natural rock formations, scrubbing the CO2 out from flue gas by re- acting it with bases, or transforming the CO2 into other useful molecules for making polymers, just to name a few. What causes smog? Smog is caused by chemical reactions involving volatile organic chemicals and nitrogen oxides that take place in the presence of sunlight. These pollutants can come from a va- riety of sources, but in urban areas, a large quantity usually comes from motor vehi- cles. These are the reasons that smog often becomes a bigger problem when there is heavy traffic and lots of sunlight. What causes acid rain? Trees and other plants are seen here, damaged or 181 killed by acid rain. Acid rain results when water in Similar to smog, acid rain is caused by the atmosphere combines with chemicals like sulfur chemical reactions of pollutants like sul- dioxide, acidifying rainwater. fur dioxide and nitrogen oxides with oxy- gen and water in the air. The acidic pollutants formed in these reactions are the cause of acid rain. In addition to envi- ronmental pollution due to human activi- ties, volcanic activity can also cause acid rain. Acid rain can be harmful to natural

plant life, farm crops, animals, and marine life. It can also be damaging to buildings, de- pending on what materials were used in their construction. What is the air quality index (AQI)? The air quality index, or AQI, is a measure describing the amount of particulate matter found in the air. Since this value can vary significantly over short periods of time, the AQI is typically reported for a given city at least once per day. The higher the AQI value, the greater the associated health concerns. Since a few different measures exist out there for characterizing the air quality, we will spare you the details of exactly how the number is calculated. It is worth paying attention to the AQI when you are considering traveling or relocating to new places. Some large cities have developed major ongoing problems with their air quality, and looking at a city’s recent AQI history is a useful in- dicator of the air quality you can expect to find. The table below summarizes relevant ranges of AQI values used in the United States and their health implications. Air Quality Index Level of Health Concern Color (AQI) 0-50 Good Green 51-100 Moderate Yellow 101-150 Unhealthy for Sensitive Groups Orange 151-200 Unhealthy Red 201-300 Very Unhealthy Purple 301-500 Hazardous Maroon What is the ozone layer and why is it so important? Ozone has the chemical formula O3, and it is a gas that is naturally present in the Earth’s atmosphere. Most of the ozone in the atmosphere exists as part of a layer that sits a few miles above the ground. This is the ozone layer, and it serves to protect the Earth from a significant fraction of the potentially harmful UV light from the Sun. Depletion of the ozone layer means that more harmful UV light reaches the Earth’s surface. Ozone is also a greenhouse gas, which means that it also plays an important role in controlling the Earth’s climate. What pollutants are dangerous to the ozone layer? The pollutants principally responsible for depletion of the ozone are volatile halogenated organic compounds. The most well known among these are chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), which used to be used in virtually all air con- ditioning, refrigeration, and cooling systems. Another is methyl bromide, which is used 182 commercially as an agricultural fumigant.

EMERGING SOURCES OF ENERGY ENERGY How do solar cells work? The Sun gives off 1,000 watts of energy per square meter of the Earth’s surface, which, if we could harness all of it, is more than enough to fulfill all of the world’s current elec- tricity needs. It’s not easy to do this, however, which is why we still rely primarily on other energy sources. A solar cell, or photovoltaic cell, harnesses energy from photons of sunlight by having them excite electrons in a material. An electric field is established within the solar cell using a process called doping, and this makes it so that the excited electrons can only flow in one direction. Thus, when sunlight strikes the solar cell, a cur- rent flows, and this is the basic principle on which solar cells operate to capture energy. The harnessed energy can either be used immediately or it can be stored for later use. Designing more efficient solar cells is a very active area of research, and you can expect their efficiency to continue to increase in coming years. How do wind turbines generate energy? Wind can be converted into energy by a wind turbine, which typically consists of a wheel with large blades connected to a series of gears. As wind causes the blades of the turbine to spin the turbine collects mechanical/kinetic energy, which can either perform me- chanical processes or be converted into electrical/potential energy. Some mechanical processes that make use of wind turbines are pumping water and grinding grain. To store electricity the turbine must be connected to a generator, which converts the mechani- cal energy from the blades into stored electrical energy. Typically, wind speeds of 7–10 mph are needed to generate energy using a wind turbine. The orientation of a wind turbine can be controlled by a computer to optimize the amount of energy it collects. What are carbon offsets? Photovoltaic cells, or solar cells, work by containing 183 materials such as mono- or polycrystaline silicon, A carbon offset involves a party committed cadmium telluride, or copper indium selenide whose to reducing emissions of greenhouse gases electrons are easily excited by photons from the sun, in order to compensate for emissions creating electricity. made elsewhere. The gases involved in- clude carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), sulfur hexaflu- oride (SF6), perfluorocarbons, and hydro- fluorocarbons. Conversion factors are used to attempt to equate the negative effects of the different gases on the atmosphere. Carbon offsets are typically purchased by either companies or governments under regulations with regard to greenhouse gas

emissions or by personal consumers who wish to offset their own contributions to green- house gas emissions. What is biodiesel? Biodiesel is a type of fuel made from vegetable oil and/or animal fat which consists of long alkyl esters (see “Organic Chemistry” to review functional groups). Biodiesel fuel is commonly distributed for sale as a mixture with petrodiesel and is labeled with a “B factor” describing the fraction of biodiesel present. B100 is pure biodiesel, while B20 would represent a mixture of 20% biodiesel with 80% petrodiesel. How is biodiesel produced? The production of biodiesel fuel involves a chemical reaction called transesterification of lipids (those from the vegetable oil or fat) with alcohols to produce alkyl esters. Methanol is most commonly used as the alcohol in these reactions, which leads to methyl esters, though other alcohols have also been used. Glycerol is a byproduct of the transesterification reaction, and this compound is ac- tually formed in fairly substantial quantities (ca. 10% by mass). This has given rise to research directed toward finding ways to carry out chemical reactions beginning with, or involving, glycerol, such that the cost efficiency of the overall process of biodiesel production might be improved. How is ethanol produced for use as a fuel? Ethanol is produced by fermenting sugars from plants like corn, soybeans, and sugar- cane. The sugars in the plant material are first broken down and then “fed” to yeast, which ferments them to produce ethanol as a byproduct. How does ethanol work as a fuel? Ethanol works as a fuel in much the same way as gasoline; it is burned in a combustion reaction to release energy. In automobiles, ethanol is usually mixed with gasoline. Most How could algae potentially be used as a source of energy? Algae are potentially a very useful source for biofuels, but currently the cost of using them as a source of biofuels is too high to make them practically use- ful. One current area of research involves using the algae biofuel production process to produce a by-product that is rich in protein and that could be used to feed farm animals. This would help to offset the cost associated with algae biofuel production. Since corn is currently used to both feed animals and to produce ethanol for fuel, the protein-rich by-product could also help by reducing the amount of corn that needs to be grown. 184

cars can run on an ethanol-gasoline mix- ENERGY ture with 10% ethanol (E10)—but to use an 85% ethanol mixture (E85) requires a specially designed system. Ethanol burns much cleaner than gasoline, making it less hazardous to the environment. It is also a renewable resource that can be produced by growing crops, so it has the potential to reduce dependence on foreign oil sources. How is hydrogen produced for use as An experimental fuel cell car is shown here. Fuel cell a fuel? technology has been deployed commercially in re- Most of the current hydrogen production cent years, but is more often seen in public vehicles in the U.S. is carried out by steam reform- such as buses than in private cars. ing natural gas (methane), which involves reacting steam with methane to generate H2. To make hydrogen a viable fuel source, more efficient ways of producing hydrogen on the large scale will be needed. Many scientists are currently investigating chemical and biological catalysts capable of carrying out a process called water splitting, which is the production of H2 and O2 from water (H2O). Water splitting may have the potential to make hydrogen into a viable fuel source for vehicles. How do hydrogen-powered cars get their energy? Hydrogen-powered cars are based on fuel cells that store hydrogen, or H2 gas, inside a material called a polymer exchange membrane. The fuel cell contains two electrodes: an anode (negative side) and a cathode (positive side). At the anode, the H2 molecules are split into protons and electrons. The protons pass through a polymer exchange mem- brane, while the electrons are unable to pass through this membrane and thus have to flow in a different direction. This creates a current of electricity by which the car is powered. What wavelengths of light from the Sun reach Earth’s surface? The light from the Sun reaching the Earth’s surface spans a range of wavelengths be- tween roughly 300 to 2,500 nanometers with a few gaps in between where atmospheric water and carbon dioxide absorb radiation. The range of wavelengths spanned by light in the Earth’s upper atmosphere is slightly broader, demonstrating that gases present in the atmosphere absorb a notable amount of light before they reach the Earth’s surface. How much energy in the United States comes from hydroelectric 185 power sources? About ten percent of the electricity in the United States comes from hydroelectric power sources. The state of Washington leads the United States in hydroelectric en-

As sunlight reaches the earth, it must penetrate the atmosphere, which absorbs or reflects much of the radiation before it reaches the surface of the planet. ergy production, with roughly 87% of this state’s electricity coming from hydroelec- tric power! What is natural gas, and where does it come from? Natural gas is primarily composed of methane (CH4), and like other hydrocarbons, methane provides energy via combustion reactions. Natural gas is typically found un- derground, near sources of petroleum, and can be pumped up via pipelines. Actually, nat- ural gas does not have a smell, so it is mixed with a small amount of a strong-smelling thiol compound so that you can tell if you have a gas leak in your home. What is fusion and how might it be used as an energy source? We looked at fusion in “Nuclear Chemistry.” To recap, fusion involves the combination of two nuclei to form a single nucleus. For lightweight nuclei, this process typically in- volves the release of energy. Unfortunately, it’s currently very difficult to make fusion happen under practical conditions; nuclei typically have to be accelerated to very, very high speeds to make fusion take place. What is a renewable energy source? A renewable energy source is one that is provided by resources that are continuously re- 186 plenished or which will always be available during the foreseeable future of the planet.

So if the radiation coming from the sun more or less covers all ENERGY wavelengths of visible light, why aren’t plants black? Let’s start with explaining the question a bit more. You can see from the graph on the preceding page that the sun provides light in almost every wavelength. The vast majority of plants, however, appear green, which means they are absorb- ing blue and red light and reflecting the green light back into your eyeballs. If plants were taking in all the energy the sun is providing, they would appear black, as no light would be reflected. So, to restate the question, why do plants reflect any light at all? The easiest answer here is that absorbing red and blue energy obviously works well enough, so evolution stuck with it. Evolution, after all, doesn’t provide the best solution, but rather a solution. Nonetheless, let’s make some guesses as to why plants are green. Chlorophyll isn’t just after energy in general, but needs very specific wavelengths to pass along energy to the reactive centers of Photosystems I and II. The second reasonable guess is that too much energy is not a good thing. Absorption of light creates high-energy species, and if they aren’t used in some productive way the energy will find something else to do. These other reactions could be destructive to cells. In sum, chlorophyll only takes what it needs or what it can use. Which energy sources are renewable? Renewable energy sources include wind, hydroelectric, solar, biomass/biodiesel, and ge- othermally derived power. What fraction of power globally comes from renewable energy sources? It is estimated that about 16% of global energy consumption currently comes from re- newable resources. Hopefully this number will increase significantly in the near future. QUANTIFYING POWER 187 What is a watt? A watt is the standard SI unit of power, representing 1 Joule of energy per second. It is named after a Scottish engineer by the name of James Watt. To give an idea of how much energy this is, lightbulbs in your house typically use about 25 to 100 watts (new types of lightbulbs are doing a bit better than this, actually). For comparison, if you are doing manual labor over the course of a long day, your body will average somewhere in the range of 75 watts of power output.